Spatial-temporal variations of microbial water quality in surface reservoirs and canals used for irrigation

► Irrigation water in reservoirs has higher microbial quality than water in ditches. ► Precipitation influences microbial water quality in canals only when over 20mm. ► Adequacy of sampling frequency required was assessed using bootstrap analysis. ► Single water test is a poor predictor of microbial quality over an irrigation season. Guidelines for microbial irrigation water quality have been proposed by regulatory agencies and organizations to reduce potential risks of waterborne contamination of produce. Most recommendations emphasize threshold values of fecal indicators and typically rely on only a limited number of water tests over the course of irrigation season. In this study, two irrigation canals and four surface reservoirs located in Ohio, USA were repeatedly sampled (n=227) to investigate fluctuations of fecal indicators concentration over an irrigation season (2010). Bootstrap analysis was applied to determine the sensitivity of the measured parameters as a function of sampling frequency. Escherichia coli counts in water collected from irrigation canals were approximately one order of magnitude higher (2.48±0.79 log MPN per 100ml) than that in reservoirs (1.54±0.04 log MPN per 100ml) and increased following heavy rainfall events (>20mm) (P